A conventional lens unit has a zoom function of driving two or more lens groups in the direction of an optical axis, as shown in FIG. 8 (see JP 2003-270509 A, for example). In FIG. 8, an imaging element is in the positive direction of the Z-axis, and a subject is in the negative direction of the Z-axis.
The lens unit having a zoom function shown in FIG. 8 has a first lens holder 51 holding a first lens group 50 and a second lens holder 53 holding a second lens group 52. The first lens holder 51 is guided in the direction of the optical axis (Z-axis) along a first guide shaft 54. The second lens holder 53 is guided in the direction of the optical axis along a second guide shaft 55. In this way, the first and second lens holders 51 and 53 can be moved in the direction of the optical axis along the first and second guide shafts 54 and 55, respectively. The first and second lens holders 51 and 53 are provided with a driving means (not shown) and can be moved in the direction of the optical axis. The first lens holder 51 and the second lens holder 53 are movable relative to each other and may therefore approach each other.
However, the lens unit having the zoom function has the following problems. A space is needed to avoid contact between the lens holders when the lens holders approach each other. Thus, it is difficult to shorten the lens unit, and the optical design needs to take it into consideration to provide this space.
Furthermore, another conventional lens unit uses a so-called hybrid lens with resin glued to a surface of it. This hybrid lens is used to increase the flexibility of an optical design and enables a design to reduce an optical total length. Thus, the total length of the lens unit can be reduced.
FIG. 9 is a cross-sectional view of the hybrid lens 69. As shown in FIG. 9, resin is applied to an optical surface 71 of a lens 70 and then shaped with a mold, so that a resin protrusion 73 is generated on a lens rim 72 around the optical surface of the lens 70. The resin protrusion 73 has a thickness in the direction of the optical axis.
There are problems as described in the following items (1) to (5) in the use of such a hybrid lens 69.
(1) Since the resin protrusion 73 has a thickness in the direction of the optical axis, it is difficult to bring other members or components close to the hybrid lens 69.
(2) Since it is difficult to control the thickness in the direction of the optical axis of the resin protrusion 73, it is necessary to keep long distances from nearby lens groups.
(3) When the resin protrusion 73 is physically removed, dust is generated and affects the optical characteristic of the lens unit.
(4) Other components should be located while avoiding the resin protrusion 73 and should be therefore downsized.
(5) If a small amount of resin is applied and molded so as not to generate a resin protrusion 73, poor molding occurs due to resin un-filling.